Various industrial products are prepared which have at least one component as a rubber composition which is based upon an EPDM or EPR rubber.
It is sometimes desired to improve the cure efficiency for peroxide curing of an EPDM-based, or EPR-based rubber composition, namely providing a faster cure system for a reduction in cure time, commensurate with a suitable processing safety, or processing without generating scorched rubber.
In practice, EPDM (ethylene/propylene/non-conjugated diene terpolymer elastomers) based rubber compositions, as well as rubbery ethylene/propylene polymer (EPR) based rubber compositions, are often cured (vulcanized) with peroxide curatives and sometimes with a combination of peroxide and a co-agent such as, for example, sulfur or acrylate co-agent.
However, it is not seen where a combination of sulfur and acrylate co-agents have contemplated for peroxide curing of EPDM or EPR elastomers.
It is to be appreciated that peroxide curatives generate free radicals for the EPDM or EPR, as the case may be, curing process which serve to crosslink the EPDM or EPR with carbon-to-carbon crosslinks. Some peroxide curatives promote faster curing of EPDM and EPR rubbers than others.
For peroxide curing of EPDM and EPR rubbers, free radicals formed during its peroxide-based curing is typically accompanied by a small amount of side reactions such as, for example, beta scission of a portion of the rubber itself which reduces the molecular weight of the polymer, and tends to degrade physical properties.
In order to minimize, or retard, the effects of such side reactions, a co-agent may be used in combination with the peroxide curative to react with the free radicals formed during the curing process and to stabilize the said radicals. In this manner, a co-agent tends to improve the overall crosslinking efficiency, thereby leading to higher cure rate and state of cure. This is well known to those having skill in such art.
Processing safety, for the purposes of this invention, relates to the processing of a rubber composition with conventional rubber mixing apparatus up to temperatures of about 130.degree. C., or sometimes even up to about 150.degree. C., for reasonable periods of time without appreciably scorching the rubber composition itself. The term "scorching" relates to prematurely curing the rubber composition while it is being mixed, or processed, in the aforesaid rubber mixer, particularly in an internal rubber mixer. The undesirable aspect of scorching, or pre-curing, of rubber during its mixing in an internal rubber mixer is well known to those having skill in such art.
Accordingly, peroxide curatives for EPDM and EPR rubbers are often selected according to their decomposition rate, namely their rate of forming free radicals during the curing of an EPDM or EPR.
For example, product manufacturing operations that desire relatively fast curing times, or relatively short molding times, typically use peroxide curatives with a relatively short half-life such as, for example, diacyl peroxides, although such peroxides may have a reduced processing safety, or a greater tendency to result in a scorched rubber composition.
On the other hand, peroxide curatives that typically exhibit slower curing times for curing EPDM's and EPM's, such as for example, di-tert-alkyl peroxides, usually exhibit good process safety, or resistance to scorching of the EPDM, or EPR, as the case may be, yet require relatively long cure times.
A philosophy of selection of peroxide curatives to balance curing speed with elimination of at least reduction of scorching of EPDM and EPR rubber compositions is well known to those having skill in such art.
In practice, sulfur is sometimes used as a co-agent for peroxide curing of EPDM's and EPR's, as is well known to those having skill in such art.
Other co-agents sometimes used as additives for peroxide-based cure systems for rubber compositions are typically polyfunctional chemicals that react readily with free radicals which are generated by the peroxide cure system. Such reaction by a co-agent tends to reduce the side reactions of free radicals generated by the peroxide curative during the curing of the EPDM rubber, including chain scission of the rubber itself. In practice, such co-agents are basically used to improve both the cure rate and the state of cure of the rubber composition.
Examples of use of sulfur and various co-agents in peroxide curing of polymers may be found, for example, in Rubber Chem. and Tech., vol 61, page 238 (1988) by R. C. Keller.
In one aspect of this invention, it is desired to provide a method for appreciably increasing a cure rate for an EPDM-based or EPM-based rubber composition while substantially retaining, or even improving, its processability, or scorch safety.
In the description of this invention, the term "phr" as used herein, and according to conventional practice, refers to parts of a respective material per 100 parts by weight of rubber. In the description herein, rubber and elastomer are used interchangeably unless otherwise noted. The terms "cure", "vulcanized" and "crosslinked" also used interchangeably unless otherwise noted. Further, the terms "uncured", "unvulcanized" and "uncrosslinked" are used interchangeably to refer to a rubber composition which has not been cured, vulcanized or crosslinked" unless otherwise noted.